Years have passed since the accident that transformed André Delambre into the fly creature. His son Philippe (Brett Halsey) learns the truth behind his father's death, and — against the urging of his uncle François (Vincent Price) — decides to continue his father's work by perfecting the teleporting device he'd worked on. However, Philippe's treacherous partner (David Frankham) seeks to steal his work, and causes another fateful teleporter accident that swaps heads with a man and a fly.
This sequel has examples of:
- Animal Testing: Philippe and co. test their disintegrator-integrator with guinea pigs and rabbits. A corpse of a policeman ends merged with the latter, which results in their front limbs changing places.
- Axe Before Entering: Barthold, Alan's partner in crime, tries to evade the fly monster by locking himself inside his morgue, but the door is soon smashed by the enraged and deranged Delambre.
- Big Bad: Alan Hinds (real name Ronald Holmes), who tries to steal Philippe's work so that he can sell it to the highest bidder.
- It Always Rains at Funerals: The film opens on the burying of late Hélène Delambre, Philippe's mother, which takes place on a rainy day.
- Revenge of the Sequel
- Teleporter Accident: After Philippe confronts Alan about his criminal doings, a fight breaks out and Alan knocks Philippe out and locks him inside the disintegrator chamber. In order to make Philippe unable to tell anyone about his crimes, he puts a fly in there as well, so that when he integrates he'll be merged with it.
- Touch of the Monster:
- Standard poster for the film shows a snarling, fanged fly monster closing on a screaming woman.
- One other poster has a gender-inverted example with the fly monster grappling with Vincent Price's character.
- The X of Y: Used for the title.